1. Field of the Invention
The present invention generally relates to thermal insulation elements and particularly to such elements having a plurality of layers of metallic material, formed to surround a component to be insulated.
2. Description of the Prior Art
Known thermal insulation assemblies are formed to have layers of plane-parallel foils or sheets having a low emission coefficient. Aluminum or steel is used as the material for the foils or sheets. The geometry of such insulation assemblies is such that a maximum heat barrier effect is obtained with low thermal conductivity and convection.
Thermal insulation assemblies are also known in which alternate layers of continuously corrugated sheets are spaced by plane sheets. The corrugated sheets in these assemblies serve solely as spacer members to prevent contact between the plane sheets. Such insulation assemblies, however, are able to absorb only limited forces and energy. Energy absorbing assemblies of metal thermal insulation elements make use of metal honeycombs or annular structures to transmit compressive forces, while the individual insulating sheets are maintained planar.
The advantages of the aforementioned metal insulation assemblies, especially where austenitic steel is used as the material, reside in their insensitivity to humidity, in their relative freedom from dust in comparison with non-metallic insulation materials, in their insolubility in liquids such as water containing boron, in the ease with which they can be decontaminated, and in their incombustibility. Their thermal expansion rate is moreover of the same order as that of the component to be insulated. It is therefore possible to arrange a gap in the assembly which will close in the normal operational state. Due to these named advantages, all-metal insulation assemblies can be used to great advantage in nuclear power station insulation and construction.
In the design of nuclear power stations special accident cases such as the rupture of a principal coolant line must be allowed for. In such designs it is presently accepted to provide deflection limiting devices for thermally insulated pipes in nuclear power stations. Such limiting devices are intended to withstand the reaction forces of the medium escaping the rupture location. These deflection limiting devices are situated at certain spacings from the insulated pipe to permit thermal expansion. The deflecting limiting devices must therefore be capable of absorbing not only the reaction forces of the escaping medium but also the impact loading caused by the accelerated pipe and the coolant which it contains. This impact loading can attain the same order of magnitude as that due to the reaction forces. Thus the deflection limiting device and its anchorage must be designed for twice the load. Where an insulation element of one of the aforedescribed types is used, the resultant damping is negligible, since the absorbed forces are only of the order of 10.sup.-4 to 10.sup.-2 times the loads which occur. The design of the deflection limiting devices and their anchorages for this double load, however, results in a considerable increase in the cost of construction.